Couldn't and wouldn't are two very different adjectives here. We aren't willing to create a proper closed ecosystem because our economic system is driven by greed and infinite growth.
Of course. But it's easy to say something is impossible to do when we don't even devote serious time and resources to doing it. Experimentally or globally with the real biosphere.
I'm not saying that it's impossible, only that we could not do it on earth, which means doing it on a much less hospitable planet is a just a pipe dream.
For now, yes. Although with the world and climate going the way it is i don't suppose it matters much what is made possible three or more decades from now.
We can continue doing this because we currently still have an abundance of resources to deplete and are convinced the problem does not and will not exist by those with money and power.